1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine or printer using an electrophotographic process or electrostatic recording process.
2. Related Background Art
In recent years, transmission of digital data information by a data communication network and an image forming apparatus as a hard output device for the information have vigorously been proposed. As this kind of image forming apparatus, a digital printer and a digital copying machine exist.
FIG. 19 shows a schematic construction of the digital printer. A photosensitive member (photosensitive drum) 1 is constructed by forming a photoconductive layer on a cylindrical conductive base member and is pivotably supported in the direction shown by an arrow R1 in the diagram. Around the photosensitive drum 1, substantially in accordance with the order along the rotating direction, a scorotron charger (primary charger) 2 for uniformly charging the surface of the photosensitive drum 1, an exposing device 3 for reading original and exposing the photosensitive drum 1 on the basis of an image signal that is proportional to the density of an image to form an electrostatic latent image, a developing device 7 for developing the electrostatic latent image as a toner image by adhering toner to it, a corona transfer charger (transfer charger) 8 for transferring the toner image formed on the photosensitive drum 1 to a transfer sheet P as a transfer material, an electrostatic separate charger (separate charger) 9 for separating the transfer sheet P on which the toner image has been transferred from the photosensitive drum 1, a cleaning device 3 for removing the residual toner on the photosensitive drum 1 after the toner image is transferred, and a pre-exposing lamp 30 for eliminating a residual charge on the photosensitive drum 1 are arranged. The transfer sheet P on which the toner image has been transferred is conveyed to a fixing device 12 after it is separated from the photosensitive drum 1. The toner image is fixed to form a desired print image. The transfer sheet P is discharged out of the image forming apparatus main body.
In a reader unit 18, light is irradiated to original 15 stacked on an original glass stage 14 by an illuminating lamp 16 and reflected light is formed as an image on a photoelectric converting element (one-line CCD) 19, thereby converting the image into an electric signal corresponding to image information. The reflected light from the original 15 irradiated by the illuminating lamp 16 is introduced by mirrors 17a, 17b, and 17c and is formed as an image on the photoelectric converting element 19 by a lens 17d. The electric signal generated by the photoelectric converting element 19 is A/D converted by an A/D converter 21 to set digital image data of eight bits (8-bit). After that, in order to convert luminance information into density information by a black signal generating circuit 22, the data is logarithmically converted and is set to image density data by a binarization circuit 23.
The 8-bit digital image data signal formed as mentioned above is inputted to a laser driving circuit 24. The laser driving circuit 24 is a well-known PWM circuit and modulates a light emitting time to turn on/off a semiconductor laser 20 in accordance with the level of the inputted image density signal.
For example, as shown in FIGS. 4A to 4C, when the image data every pixel is inputted in the scanning direction of the laser as shown in FIG. 4A, a driving signal to turn on/off the laser is as shown in FIG. 4B. That is, when the image data is set to 00 hex (hexadecimal), the ON duty of the laser driving signal is set to 5% of one pixel scanning time and, when it is set to FFhex, the ON duty of the laser driving signal is set to 85% of one pixel scanning time. In this manner, dark and light states are realized by making an area gradation in one pixel.
Further, FIG. 6 shows ordinary I-L characteristics (driving current--light amount characteristics) of the laser. The driving currents which are used at the ON/OFF times of the laser are shown by Ion and Ioff, respectively. Therefore, the laser driving current for the image signal in FIG. 4 is as shown in FIG. 4C. It is a current whereby the PWM circuit drives the laser.
The laser driving system is mainly divided to the above-mentioned PWM circuit and a binary laser driving circuit. As mentioned above, the PWM circuit modulates the signal to a pulse width signal corresponding to the time for light emission by the semiconductor laser in accordance with the level of the inputted image density signal. On the other hand, the binarization circuit converts the signal into a two-stage signal of a specific ON light emission signal and an OFF signal corresponding to a pixel size and inputs the converted signal to the laser driving circuit 24, thereby turning on/off the laser (semiconductor laser device) 20. As a representative binarizing method, there is a method of generating a binarized signal on the basis of image data by using a method such as error diffusion method or dither method. Fundamentally, the time to generate laser light is held constant irrespective of the density. It differs with respect to a point that the laser light is emitted to a pixel having a low density at a low probability and, as the density of a pixel is higher, the laser light is emitted at a higher probability.
The laser light driven and emitted in accordance with the image signal as mentioned above is written to the photosensitive drum 1 in a raster scanning manner through a polygon mirror scanner 28 that is rotated at a high speed and a mirror 17f to form a digital electrostatic latent image as image information.
The electrostatic latent image on the photosensitive drum 1 is developed to be visualized by the developing device 7 by using the toner.
The toner image on the photosensitive drum 1 is electrostatically transferred to the transfer material P by the transfer charger 8.
The separate charger 9 applies a high AC or DC superposed voltage having a polarity opposite to that in the transfer to a conductive wire that is stretched in a metallic shield case having a low potential, biased bipolar ions are supplied to the rear surface of the transfer material which faces the drum to substantially neutralize and reduce a transfer electric field, and the electrostatic adsorption charge is eliminated from the transfer material electrostatically absorbed to the surface of the drum. Consequently, the transfer material is separated from the curving surface of the drum due to the self-weight and the rigidity of the transfer material. As described in the transfer as well, the transfer current is increased in order to assure the transfer efficiency by the toner having low transfer efficiency. It is disadvantageous for separation performance and it is difficult to assure the absolute amount of the charge eliminating current and balance the two polarities.
Particularly with respect to the separation of the transfer material, as the operating speed of the image forming apparatus is faster, the charge eliminating current is lacked, so that the separation becomes unstable.
In order to cope with the above, means for increasing the applied voltage is used as a method of remarkably increasing the total current amount for the separation. However, when the voltage is more than enough, a current leakage easily occurs between the photosensitive drum and the separation charge shield portion. Particularly, in the case where the apparatus has been used for a long time, paper particles of the transfer material and scattered toner are deposited on the shield of the separate charger and stain the shield. Consequently, unevenness as discharge impedance occurs in a low potential shield portion as a discharge counter electrode, the charges concentratedly flow into a low impedance portion, and the leak occurs extremely often. Further in a high humidity environment, a material which stains the shield absorbs moisture. The leak easily occurs due to the moisture absorption by, especially, the long fiber-shaped paper particles.
Particularly, the leak easily occurs in a non-image area when it is necessary to operate the separate charger in the non-image area where no transfer material exists for a period of time during which the image is not formed. When no transfer material exists and the current cannot flow into the transfer material, the leak easily occurs.
When the leak current is large, as a derived failure, such a failure that a pit is formed on the photosensitive drum due to a voltage attack at the leak time to generate a black dot or the discharging wire of the separate charger is disconnected occurs.